1. Field of the Invention
The present invention relates to an optical pickup device and an optical recording medium driving apparatus comprising at least an objective lens having a numerical aperture (NA) of 0.75 or more, and an aberration correction element for correcting a sphere aberration generated by a difference of a substrate thickness between first and second optical recording mediums at a time when the objective lens is used, to selectively record on or reproduce from the first and second optical recording mediums different in substrate thickness using first and second laser lights having different wavelengths.
2. Description of the Background Art
In general, optical recording mediums such as a disc-shaped optical disc and card-shaped optical card have been frequently used, because the mediums are capable of recording information signals such as video information, sound information, and computer data in tracks spirally or concentrically formed in a transparent substrate, and capable of accessing a desired track at a high rate during reproduction of recorded tracks.
As this type of optical recording medium, the optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have already been on the market. In recent years, an extra-high density optical disc (Blu-ray Disc) has been actively developed which is capable of recording or reproducing information signals at a density higher than that of CD, DVD in order to increase the density of the optical disc.
First, for the above-described CD, a disc substrate has heretofore been irradiated with a laser beam obtained by focusing a laser beam having a wavelength of around 780 nm with an objective lens having a numerical aperture (NA) of about 0.45 to record or reproduce the information signal on or from a signal surface which is positioned distant from the laser incident incidence surface of the disc substrate by approximately 1.2 mm.
Moreover, for the above-described DVD, the disc substrate has heretofore been irradiated with the laser beam obtained by focusing a laser beam having a wavelength of around 650 nm with an objective lens having a numerical aperture (NA) of about 0.6 to record or reproduce the information signal on or from the signal surface which is positioned distant from the incidence surface by approximately 0.6 mm. In this case, a recording capacity of the DVD is increased six to eight times that of the CD, and the capacity of one surface is about 4.7 gigabyte (GB) with a diameter of a disc substrate of 12 cm.
Furthermore, the above-described extra-high density optical disc has been developed in which the disc substrate is irradiated with a laser beam having a wavelength of 450 nm or less, which has been focused with an objective lens having a numerical aperture (NA) of 0.75 or more, so as to be capable of recording or reproducing the information signal on or from a signal surface which is positioned distant from the incidence surface by approximately 0.1 mm. In this case, the recording capacity of the surface of the extra-high density optical disc is around 25 gigabytes (GB) with the diameter of the disc substrate of 12 cm.
Additionally, there has been an optical pickup device comprising an objective lens shared with respect to two types of optical discs already on the market such as the CD and DVD and an optical element which transmits all the laser beams having a wavelength of around 650 nm to record on or reproduce from the DVD while interrupts the laser beam having a wavelength of around 780 nm by an outer circular portion thereof and diffracts it by an inner circular portion thereof to record on or reproduce from the CD (see, for example, Japanese Patent Application Laid-Open No. 2000-105943 (pages 4 to 6, FIGS. 1, 10, 11)).
FIG. 1 is a constitution diagram showing one example of a conventional optical pickup device. FIGS. 2A and 2B are explanatory views of a second optical element shown in FIG. 1, where FIG. 2A is a diagram showing the recording or reproducing of the DVD, and FIG. 2B is a diagram showing the recording or reproducing of the CD.
A conventional optical pickup device 110 shown in FIGS. 1, 2A, and 2B is described in the Japanese Patent Application Laid-Open No. 2000-105943. The device will be briefly described with reference to the document.
As shown in FIG. 1, the conventional optical pickup device 110 comprises: a first semiconductor laser 111 which emits a laser light La having a wavelength of 650 nm for a DVD 101; a second semiconductor laser 112 which emits a laser light Lb having a wavelength of 780 nm for a CD 102; a first optical element 113 which corrects an optical axis deviation between the laser light La having a wavelength of 650 nm and the laser light Lb having a wavelength of 780 nm; a correction plate 114 which corrects a phase difference caused by the first optical element 113; a half mirror 115 which transmits the respective laser lights La, Lb emitted from the first and second semiconductor lasers 111, 112 and which reflects each light reflected by a signal surface 101a of the DVD 101 or a signal surface 102a of the CD 102 so as to change a direction of the light by approximately 90°; a collimator lens 116 which forms the laser light La having a wavelength of 650 nm and the laser light Lb having a wavelength of 780 nm into parallel lights; a second optical element 117 which allows the laser light La having a wavelength of 650 nm, transmitted through the collimator lens 116, to be incident upon an objective lens 118 and which interrupts the laser light Lb having a wavelength of 780 nm, transmitted through the collimator lens 116, by the outer circular portion thereof and diffracts the light by the inner circular portion thereof and allows the light to be incident upon the objective lens 118; the objective lens 118 which focuses the laser light La having a wavelength of 650 nm and the laser light Lb having a wavelength of 780 nm to converge the light on the signal surface 101a of the DVD 101 or the signal surface 102a of the CD 102; a Wollaston prism 119 which separates each reflected light from the DVD 101 or the CD 102 into a laser light containing only an s-polarized component, a laser light containing only a p-polarized component, and a laser light containing the s-polarized component mixed with the p-polarized component; and a photodetector 120 which detects the laser light of only the s-polarized component, the laser light of only the p-polarized component, and the laser light of the s-polarized component mixed with the p-polarized component from the Wollaston prism 119.
Here, in the above-described constituting members, as enlarged and shown in FIGS. 2A and 2B, especially the second optical element 117 comprises an outer circular portion 117a and an inner circular portion 117b. Moreover, the outer circular portion 117a of the second optical element 117 has a function of transmitting the laser light La having a wavelength of 650 nm wholly as it is, and diffracting only the laser light Lb having a wavelength of 780 nm outside an optical axis to prevent the light from being incident upon the objective lens 118. On the other hand, the inner circular portion 117b of the second optical element 117 has a function of transmitting the laser light La having a wavelength of 650 nm wholly as it is, and diffracting only the laser light Lb having a wavelength of 780 nm to allow the light to be incident upon the objective lens 118.
In this case, for a sectional structure of the second optical element 117, the outer circular portion 117a includes a concave/convex pattern structure, the inner circular portion 117b roughly has triangular shapes, and a portion corresponding to a slope of each triangular shape has a stair-like pattern structure.
Moreover, as shown in FIG. 2A, in the recording or playback of the DVD 101 by the objective lens 118, the laser light La having a wavelength of 650 nm passes as such without being influenced by the second optical element 117 and is incident upon the objective lens 118. The light is focused onto the signal surface 101a of the DVD 101 having a disc substrate thickness of 0.6 mm by the objective lens 118.
On the other hand, as shown in FIG. 2B, in the recording or playback of the CD 102 by the objective lens 118, for the laser light Lb having a wavelength of 780 nm, a portion incident upon the outer circular portion 117a of the second optical element 117 is largely diffracted outside the optical axis by a concave/convex diffraction pattern, and is not incident upon the objective lens 118. The laser light Lb incident upon the inner circular portion 117b of the second optical element 117 is diffracted to the outside by the triangular stair-like pattern structure, but is incident upon the objective lens 118 without being largely diffracted as in the outer circular portion 117a. Therefore, only the laser light Lb incident upon only the inner circular portion 117b of the second optical element 117 reaches the objective lens 118, and is focused onto the signal surface 102a of the CD 102 having a disc substrate thickness of 1.2 mm by the objective lens 118.
That is, the laser light Lb having a wavelength of 780 nm is substantially interrupted by the outer circular portion 117a of the second optical element 117, diffracted by the inner circular portion 117b, and is incident upon the objective lens 118. In this case, the objective lens 118 is designed for the DVD having a disc substrate thickness of 0.6 nm. Therefore, when the laser light Lb having a wavelength of 780 nm is only interrupted by the outer circular portion and is incident upon the disc substrate having a thickness of 1.2 mm, aberration is generated, and the lens is designed so as to reduce this aberration. Therefore, a diameter of the inner circular portion 117b of the second optical element 117, and a size of the triangular shape which causes diffraction are determined in such a manner that an effective numerical aperture of the objective lens 118 is 0.45 with respect to the laser light Lb having a wavelength of 780 nm. It has also been described that when the signal recording into a recordable/reproducible CD-recordable (CD-R) is considered, an effective numerical aperture of the objective lens 118 is suitable, and therefore the diameter of the inner circular portion 117b of the second optical element 117 is determined so as to set the effective numerical aperture of the objective lens 118 to 0.45 to 0.50.
Additionally, according to the conventional optical pickup device 110, the DVD 101 and the CD 102 can be selectively recorded or reproduced using the objective lens 118 designed for the DVD and the first and second optical elements 113, 117. However, it is clear that anything is not considered with respect to the extra-high density optical disc being developed in the conventional optical pickup device 110.
On the other hand, when proceeding with the development of the extra-high density optical disc, needless to say, there has also been a demand for a recordable or reproducible optical pickup device and optical disc driving apparatus in which an extra-high density optical disc and DVD are used in common. Furthermore, there is also a possibility of a recordable or reproducible optical pickup device and optical disc driving apparatus in which an extra-high density optical disc and CD are used in common.